1. Field of the Invention
The present invention relates to a technique suitable for application to a position detecting device, and particularly to a technique for improving scanning speed in a position detecting plane of a capacitance type position detecting device.
2. Description of the Related Art
There are various input devices for supplying positional information to a computer. Among the input devices, there is a position detecting device referred to as a touch panel.
A touch panel is a position detecting device allowing operation of a computer or the like by touching a detecting plane with an indicating object such as a finger, a dedicated stylus, or the like. The touch panel is widely used in PDAs (Personal Digital Assistants), ATMs (Automated Teller Machines) in banks, ticket vending machines in railroad stations, and the like.
There are various positional information detecting techniques employed in the touch panel. These are, for example, a resistive film system that detects a position on the basis of a change in pressure applied against a position detecting plane, a capacitance system that detects a position on the basis of a change in capacitance of a film in a surface of a position detecting plane, and the like.
Description will be made of a position detecting device based on the capacitance system as a conventional technique of the present invention.
FIG. 15 is a block diagram showing a conventional capacitance type position detecting device.
A driving section 1502 generates an alternating voltage of 200 kHz, which frequency is considered to be most readily absorbed by a human body, for example. The alternating voltage of 200 kHz generated by the driving section 1502 is selectively applied to electrodes 209 in an X-axis direction (which electrodes will hereinafter be referred to as “X-axis electrodes”) in a sensor substrate 203 through a transmission selecting switch 1503.
The sensor substrate 203 has capacitors formed by arranging electrodes of long and narrow conductors vertically and horizontally and interposing an insulating sheet substantially in the form of a plate, which insulating sheet is not shown in the figure, between the electrodes arranged vertically and the electrodes arranged horizontally. The alternating voltage of 200 kHz is applied to these capacitors.
A reception selecting switch 1504 is a switch for determining an intersection forming a capacitor.
An output from the reception selecting switch 1504 is supplied to a preamplifier 1505, converted into digital data by an A/D converter 206, and then input to a position calculating section 207a. 
Upon receiving address information obtained from a synchronizing clock generating section 1506 and the data on slight signal change, which data is obtained from the A/D converter 206, the position calculating section 207a formed by a microcomputer outputs information on the presence or absence of a finger on the sensor substrate 203 and positional information of the finger. Specifically, the position calculating section 207a subjects the data obtained from the A/D converter 206 to integration processing, and thereafter detects a peak value thereof. The position calculating section 207a then calculates a center of gravity on the basis of the calculated peak value and values preceding and succeeding the peak value. The position calculating section 207a then calculates the position of the finger on the basis of the position of the obtained center of gravity on a time axis.
Incidentally, for the convenience of description, the plurality of electrodes connected to the transmission selecting switch 1503 in the sensor substrate 203 will hereinafter be referred to collectively as an X-axis electrode 209, and the plurality of electrodes connected to the reception selecting switch 1504 in the sensor substrate 203 will hereinafter be referred to collectively as a Y-axis electrode 210.
Description will next be made of internal parts of the driving section 1502. The driving section 1502 includes a clock generator 1507, a readout section 1508, a sine wave ROM 1509, a D/A converter 1510, a low-pass filter (LPF) 1511, and a driver 1512.
The clock generator 1507 is an oscillator for generating a clock. The clock generated by the clock generator 1507 is supplied to the readout section 1508.
The sine wave ROM 1509 is a ROM (Read Only Memory) storing a pseudo sine wave of 8 bits×256 samples, for example. The readout section 1508 specifies an address in the sine wave ROM 1509 and reads out data on the basis of the clock supplied from the clock generator 1507.
The data read out from the sine wave ROM 1509 by the readout section 1508 is subjected to D/A conversion in the D/A converter 1510, and then input to the LPF 1511 to be smoothed in the LPF 1511. The data is thereby converted into an analog sine wave signal. The analog sine wave signal is subjected to voltage amplification in the driver 1512 to become the alternating voltage applied to the X-axis electrode 209.
Incidentally, conventional techniques related to the invention of the present applicant are shown in U.S. Pat. No. 5,861,875 (hereinafter referred to as Patent Document 1) and Japanese Patent Laid-Open No. Hei 10-20992 (hereinafter referred to as Patent Document 2).